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From:   Tomasz Figa <tfiga@chromium.org>
Date:   Fri, 25 Jan 2019 12:29:11 +0900
Message-ID: <CAAFQd5DXWc1mBYZ=2zH0fPitLc=L2UGem85wokB6oht4CxVrsQ@mail.gmail.com>
Subject: Re: [PATCH v2 2/2] media: docs-rst: Document memory-to-memory video
 encoder interface
To:     Nicolas Dufresne <nicolas@ndufresne.ca>
Cc:     Hans Verkuil <hverkuil@xs4all.nl>,
        Linux Media Mailing List <linux-media@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        =?UTF-8?B?UGF3ZcWCIE/Fm2NpYWs=?= <posciak@chromium.org>,
        Alexandre Courbot <acourbot@chromium.org>,
        Kamil Debski <kamil@wypas.org>,
        Andrzej Hajda <a.hajda@samsung.com>,
        Kyungmin Park <kyungmin.park@samsung.com>,
        Jeongtae Park <jtp.park@samsung.com>,
        Philipp Zabel <p.zabel@pengutronix.de>,
        Tiffany Lin <tiffany.lin@mediatek.com>,
        Andrew-CT Chen <andrew-ct.chen@mediatek.com>,
        Stanimir Varbanov <stanimir.varbanov@linaro.org>,
        Todor Tomov <todor.tomov@linaro.org>,
        Paul Kocialkowski <paul.kocialkowski@bootlin.com>,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        dave.stevenson@raspberrypi.org,
        Ezequiel Garcia <ezequiel@collabora.com>,
        Maxime Jourdan <maxi.jourdan@wanadoo.fr>
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On Fri, Jan 25, 2019 at 5:04 AM Nicolas Dufresne <nicolas@ndufresne.ca> wro=
te:
>
> Le mercredi 23 janvier 2019 =C3=A0 12:28 +0100, Hans Verkuil a =C3=A9crit=
 :
> > On 01/23/19 11:00, Tomasz Figa wrote:
> > > On Sat, Nov 17, 2018 at 8:37 PM Hans Verkuil <hverkuil@xs4all.nl> wro=
te:
> > > > On 11/17/2018 05:18 AM, Nicolas Dufresne wrote:
> > > > > Le lundi 12 novembre 2018 =C3=A0 14:23 +0100, Hans Verkuil a =C3=
=A9crit :
> > > > > > On 10/22/2018 04:49 PM, Tomasz Figa wrote:
> > > [snip]
> > > > > > > +      rely on it. The ``V4L2_BUF_FLAG_LAST`` buffer flag sho=
uld be used
> > > > > > > +      instead.
> > > > > >
> > > > > > Question: should new codec drivers still implement the EOS even=
t?
> > > > >
> > > > > I'm been asking around, but I think here is a good place. Do we r=
eally
> > > > > need the FLAG_LAST in userspace ? Userspace can also wait for the=
 first
> > > > > EPIPE return from DQBUF.
> > > >
> > > > I'm interested in hearing Tomasz' opinion. This flag is used alread=
y, so there
> > > > definitely is a backwards compatibility issue here.
> > > >
> > >
> > > FWIW, it would add the overhead of 1 more system call, although I
> > > don't think it's of our concern.
> > >
> > > My personal feeling is that using error codes for signaling normal
> > > conditions isn't very elegant, though.
> >
> > I agree. Let's keep this flag.
>
> Agreed, though a reminder of the initial question, "do we keep the EOS
> event ?", and I think the event can be dropped.
>

I would happily drop it, if we know that it wouldn't break any
userspace. Chromium doesn't use it either.

Best regards,
Tomasz

> >
> > Regards,
> >
> >       Hans
> >
> > > > > > > +
> > > > > > > +3. Once all ``OUTPUT`` buffers queued before the ``V4L2_ENC_=
CMD_STOP`` call and
> > > > > > > +   the last ``CAPTURE`` buffer are dequeued, the encoder is =
stopped and it will
> > > > > > > +   accept, but not process any newly queued ``OUTPUT`` buffe=
rs until the client
> > > > > > > +   issues any of the following operations:
> > > > > > > +
> > > > > > > +   * ``V4L2_ENC_CMD_START`` - the encoder will resume operat=
ion normally,
> > > > > >
> > > > > > Perhaps mention that this does not reset the encoder? It's not =
immediately clear
> > > > > > when reading this.
> > > > >
> > > > > Which drivers supports this ? I believe I tried with Exynos in th=
e
> > > > > past, and that didn't work. How do we know if a driver supports t=
his or
> > > > > not. Do we make it mandatory ? When it's not supported, it basica=
lly
> > > > > mean userspace need to cache and resend the header in userspace, =
and
> > > > > also need to skip to some sync point.
> > > >
> > > > Once we agree on the spec, then the next step will be to add good c=
ompliance
> > > > checks and update drivers that fail the tests.
> > > >
> > > > To check if the driver support this ioctl you can call VIDIOC_TRY_E=
NCODER_CMD
> > > > to see if the functionality is supported.
> > >
> > > There is nothing here for the hardware to support. It's an entirely
> > > driver thing, since it just needs to wait for the encoder to complete
> > > all the pending frames and stop enqueuing more frames to the decoder
> > > until V4L2_ENC_CMD_START is called. Any driver that can't do it must
> > > be fixed, since otherwise you have no way to ensure that you got all
> > > the encoded output.
> > >
> > > Best regards,
> > > Tomasz
> > >
>